Recent developments in biotechnology have led to the discovery of various nucleic acid molecules exhibiting intracellular RNA interference functions. For example, siRNA (small interfering RNA) is known by its ability to cause a degradation of the mRNA of a target gene in a cell, thereby inhibiting the expression of the target gene (RNA interference). This function of inhibiting the expression of the target gene due to RNA interference is effective for alleviating or treating disease presentations caused by abnormal expression of specific genes or gene clusters; therefore, the development of therapeutic agents using siRNA is expected. However, siRNA and other gene therapies pose a problem in that, because the nucleic acid molecule is a water-soluble, negatively charged polymer, it has an extremely low intracellular gene delivery efficiency, resulting in an inefficient therapeutic effect.
The use of a carrier (vector) is known to efficiently deliver genes into cells. Vectors are classified into viral and nonviral vectors. In spite of their high nucleic acid introduction efficiency, viral vectors have some safety concerns including pathogenicity, immunogenicity, and cytotoxicity. Therefore, the use of non-viral vectors is desired for clinical usage.
Examples of nonviral vectors include Lipofectamine™2000, which is already commercially available, a cationic lipid (see Patent Document 1) having a specific structure, and a composition (see Patent Document 2) containing an amphiphilic compound and polycation. Delivery of a nucleic acid molecule into a cell using a nonviral vector is performed by mixing a nucleic acid molecule with a nonviral vector to form a complex, and contacting the complex with the target cell. When the nonviral vector is capable of forming liposome, the vector is incorporated into a cell with a nucleic acid encapsulated in the liposome, thereby conducting intracellular nucleic acid delivery.
However, nucleic acid molecules capable of RNA interference, such as siRNA, have a particular characteristic; they are unstable and highly negatively charged. Therefore, the stability is problematically reduced when the nucleic acid molecule is mixed with a cationic vector as a nonviral vector due to charge neutralization, which hinders the continuous delivery of nucleic acid molecules into a cell. Although an example in which a nucleic acid is entrapped in a liposome by forming a complex of the siRNA and the cationic polymer is known (see Non-patent Document 1), its practical effectiveness has not been confirmed in terms of the cytotoxicity of the cationic polymer. Moreover, although known nonviral vectors can form stable complexes with nucleic acid molecules, the problem of insufficient delivery performance into a cell still remains, or, even when delivery is successful, the retention time of the complex in the cell is short. For these reasons, the known nonviral vectors have a defect in that they cannot maintain a nucleic acid molecule in the cell, hindering constant provision of the desired effects of the nucleic acid molecule.
In view of such prior art circumstances, there has been a demand for a technique that ensures high safety and low toxicity for delivering nucleic acid molecules, for example siRNA, capable of RNA interference or translation inhibition into a cell, and continuously maintaining the nucleic acid molecules therein.